The present invention generally relates to a method of measuring the quality properties of paper and/or board on moving webs. In particular, the present invention relates to using optical contactless methods in a continuous optical spectra, preferably in the infra-red range, for measuring and evaluating the quality properties of paper and/or board on moving webs by chemometric methods.
In DE 198 30 323 A1, a method and an associated apparatus for determining the thickness of paper or board on a moving web are described in which measurements are made optically without contact and the spectra are evaluated by chemometric methods. DE 198 30 343 A1 discloses a thickness measuring method and associated apparatus which use chemometric evaluation of the optical spectral lines obtained on one side of the web from a light source on the opposite side of the web. The thickness of the web is determined by comparing training sets for a number of different material characteristics. The spectral lines can be detected via an on-line spectrometer with at least one light source on one side of the web and a number of detectors on the opposite side for detecting the transmitted light components, e.g. in the infra-red range.
The present invention provides a method for registering the properties of paper or board. The present invention utilizes a staged evaluation method. In the staged evaluation method, first, the basic properties of the paper or the board are determined and then further properties are determined via multi-stage modeling. In an exemplary embodiment, the basic properties determined are preferably: the grammage, the moisture and/or the thickness (caliper) of the paper or board. It is also preferable to perform three-stage modeling where the material composition, freeness and further individual variables are determined one after another.
Using the multi-stage modeling, the present invention determines and evaluates all variables measured for the interested quality properties of the paper or board without traversing, by means of suitable on-line modeling, unlike that disclosed in DE 198 30 323 A1 which only determined paper thickness.